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ActiveSync on iPhone does not work on AT&T, works on Verizon and Wifi

Posted on 2012-09-10
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Last Modified: 2013-08-13
We recently switched both to a new load balancing router (PepLink Balance 580) and added a new T3 line.

Upon the change, iPhones using AT&T only cannot sync via ActiveSync over 3G/4G. They do sync on Wifi (from any Wifi enabled area - not just on the LAN), and Verizon/Sprint phones can sync over 3G or Wifi.

https://www.testexchangeconnectivity.com/ works with no errors. The DNS records appear correct, as I can get to another internal site by the same URL.

The CAS is Exchange 2007 running on Windows 2003.

I am at a complete loss as to why one carrier only would have problems.
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Question by:24SevenInc
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by:joelsplace
ID: 38384786
It sounds to me like AT&T's DNS servers are having problems.  Try using the IP to connect and see if that fixes it.  If it does you know it's their DNS.
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by:24SevenInc
ID: 38384800
No such luck. I was hoping so, but every DNS app I found confirms the lookup properly, and I can get to a site at the same URL on a different port from mobile Safari.
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by:joelsplace
ID: 38384823
Do they have a port scanner app you could try like GRC does for the PC?
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by:24SevenInc
ID: 38384856
Nothing I've found.
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by:joelsplace
ID: 38384881
I would say that AT&T was blocking an activesync port but I have at least 100 users on activesync with AT&T that are working in the DFW area and they travel all over the country and never complain about it.  I do have a lot of issues setting up new iPhones.  I can have 3 side by side and punch in all the same info and 2 will connect and one won't.  Sometimes I can just punch in the info again and it will work, sometimes it takes a reboot and sometimes I have to reset the phone.  Do you have any non iPhones you can try to see if it's an iPhone specific issue?
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by:24SevenInc
ID: 38386779
I am searching for an AT&T Android we can test with, and an AT&T wireless device as well.

I don't see any routing issues to AT&T networks or anything like that.
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by:joelsplace
ID: 38386859
Have you tried using a telnet app to check the ports?  You could first try it on a working phone to verify the app works and then try a non working phone.
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by:joelsplace
ID: 38386868
I know you said this https://testexchangeconnectivity.com works.  Did you run it from the iPhone?  (never tried this myself)
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Author Comment

by:24SevenInc
ID: 38386922
I hadn't thought of Telnet - I can connect over port 443 from the phone.

https://testexchangeconnectivity.com runs from their own server, not the local device - I checked the logs. I have run ActiveSync tester on the iPhone and I'm getting a timeout over 3G.
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by:24SevenInc
ID: 38387747
I have confirmed that ActiveSync is working on an AT&T android phone.
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by:joelsplace
ID: 38388065
Do you have some special app on the iPhone that may be causing trouble with port 443?  (since we know it's an iPhone issue)
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by:24SevenInc
ID: 38388086
Nothing comes to mind - and since it's only a problem over 3G and OK over Wifi, I'd think DNS, but I can resolve it correctly by testing another port. (RDP to the same domain name from the phone over 3G works)
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by:joelsplace
ID: 38388095
That's true unless there is some kind of app that is only active when it's on 3G.  Do you have a phone you can format and try fresh?
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Expert Comment

by:Brad Bouchard
ID: 38393062
Check to ensure that AT&T processes/allows (lack of better term here) your CA's certs used to encrypt Activesync.  More plainly, make sure that AT&T accepts the cert you bought through a CA like Entrust, VeriSign, Comodo, etc.

I've heard/read of problems with all three (the Root, Intermediate, actual cert) for some providers.
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by:joelsplace
ID: 38393122
I wouldn't put it past AT&T to be blocking things due to an account screw up.  They have always told me that I have to have "enterprise data" to connect to active sync but I haven't ever paid for it and it works fine for me anyway.  I wonder if they have something messed up in your account settings?  In my experience iPhones don't care about messed up certs.  They just complain and you have to tell them it's ok.
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by:Brad Bouchard
ID: 38395766
In my experience iPhones don't care about messed up certs.

It's not that they don't care about them, it's that their list on some models and versions of iOS didn't contain certain 3rd party CAs root and intermediate certs list.
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24SevenInc earned 0 total points
ID: 38396473
The vendor correctly identified the problem. The default MTU on the device was 1440, which was causing fragmented packets. Perhaps GSM networks have lower tolerances, which allowed it to work on CDMA and straight wifi.

Setting the MTU to 1500 solved the problem.
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by:Brad Bouchard
ID: 38396689
Please provide the solution so we will know for future reference and for resolution of this question.
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Expert Comment

by:Brad Bouchard
ID: 38396779
The vendor correctly identified the problem. The default MTU on the device was 1440, which was causing fragmented packets. Perhaps GSM networks have lower tolerances, which allowed it to work on CDMA and straight wifi.

Setting the MTU to 1500 solved the problem.

Thank you so much for letting us know.
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Author Closing Comment

by:24SevenInc
ID: 38408767
Vendor provided solution.
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Expert Comment

by:x7erra37
ID: 39403927
Where did you change the MTU to 1500? I am have the same problem with ATT ActiveSync over cellular network but no problems on wifi and Verizon/Sprint carriers.
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